The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for being mounted in a relatively small camera such as a vehicle onboard camera, a digital still camera, a security camera, and a network camera.
In these years, some vehicles are equipped with a plurality of cameras for a purpose of enhancing convenience and security. For example, in case of a vehicle equipped with a backup camera to take an image behind the vehicle, a driver can see the rear view of the vehicle on a monitor upon backing up the vehicle. Accordingly, it is possible to safely move the vehicle backward without hitting an object even if such an object is not visible from the driver due to shadow of the vehicle.
Such a camera mounted on a vehicle, i.e., a so-called onboard camera, has been increasingly applied each year. In these days, there has been available a system that can process images taken by onboard cameras, which are wide-angle respectively attached on four positions of a vehicle (backdoor, front grill, and side mirrors), so as to be able to display an image of the vehicle as if viewed from above. With increase of application of such onboard cameras, the onboard cameras are required to have an ability of imaging in wider range with high resolution. Therefore, in addition to a small size thereof, an imaging lens for mounting in an onboard camera is required to have a wide angle of view and an ability of satisfactorily correct aberrations. However, in case of an imaging lens, it is difficult to achieve downsizing while satisfactorily correct aberrations and increase the angle of view, i.e., widening of an angle of view. Upon actual designing of an imaging lens, it is critical to achieve those demands in a well-balanced manner.
As an imaging lens having a wide angle of view, for example, there is an imaging lens described in Patent Reference. The imaging lens includes in the order from an object side: a first lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to the object side; a second lens that is similarly negative and has a shape of a meniscus lens directing a convex surface thereof to the object side; a stop; a third lens having a biconvex shape; a fourth lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to the object side; and a fifth lens having a biconvex shape. According to the configuration, by restraining within a certain range a ratio between the distance from a second principal point of a front group, which is composed of the first lens and the second lens, to the stop and the distance from the second principal point of the front group to a first principal point of a rear group, which is composed of the third lens to the fifth lens, it is achievable to obtain downsizing and satisfactory correction of aberrations.    Patent Reference: Japanese Patent Application Publication No. 2003-307674
According to the imaging lens described in Patent Reference, it is achievable to obtain a wide angle of view, although the number of lenses that compose the imaging lens is as few as five, and to obtain relatively satisfactorily correction of aberrations. However, since a total length of the lens system is long, it does not meet demands for downsizing in these years, and there still remain some challenges for attaining both downsizing of the imaging lens and satisfactory aberration correction. Here, such issue is not specific to an imaging lens for mounting in an onboard camera, but is common for imaging lenses for mounting in a relatively small-sized camera, such as a digital still camera, a security camera, and a network camera.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that has a wide angle of view and can satisfactorily correct aberrations in spite of a small size thereof.